5.2.6 Octahedral & Tetrahedral Complexes

• Octahedral complexes are formed when a central metal atom or ion forms six coordinate bonds
• This could be six coordinate bonds with six small, monodentate ligands
  • Examples of such ligands are water and ammonia molecules and hydroxide and thiocyanate ions
  • As there are six ligands, these complexes are sometimes described as having six-fold coordination

Table Showing Examples of Common Monodentate Ligands

Example of an octahedral complex with monodentate ligands

• It could be six-coordinate bonds with three bidentate ligands
  
  • Each bidentate ligand will form two coordinate bonds, meaning six-coordinate bonds in total
  • Examples of these ligands are 1,2-diaminoethane and the ethanedioate ion

Example of an octahedral complex with bidentate ligands

• It could be six-coordinate bonds with one multidentate ligand
  • The multidentate ligand, for example, EDTA^4-_, forms all six-coordinate bonds

Example of an octahedral complex with a polydentate ligand 

• The bond angles in an octahedral complex are 90^o
• The coordination number of a complex is the number of dative bonds formed between the central metal ion and the ligands
  • Since there are 6 dative bonds, the coordination number for the complex is 6

Changes to the central metal ion

• The main example of the central metal ion changing oxidation state is when Fe²⁺ is oxidised to form Fe³⁺ 
  • Fe²⁺ → Fe³⁺ + e^– 
• Hexaaquairon(II) is a pale green solution, which slowly oxidises on standing in air to give a yellow solution of hexaaquairon(III) ions. 
  • [Fe(H₂O)₆]²⁺ → [Fe(H₂O)₆]³⁺  
• The same oxidation reaction can be achieved with different iron(II) complex ions
  • These are often performed by bubbling oxidising agents, such as chlorine gas, through a solution of the iron(II) complex ion
  • For example, bubbling chlorine gas through a yellow solution of potassium hexacyanoferrate(II), K₄[Fe(CN)₆], results in the formation of a red solution of potassium hexacyanoferrate(III), K₃[Fe(CN)₆]
  • 2K₄[Fe(CN)₆] + Cl₂ → 2K₃[Fe(CN)₆] + 2K⁺ + 2Cl^–

Tetrahedral complexes

• When there are four coordinate bonds the complexes often have a tetrahedral shape
  • Complexes with four chloride ions most commonly adopt this geometry
  • Chloride ligands are large, so only four will fit around the central metal ion
• The bond angles in tetrahedral complexes are 109.5^o

Example of a tetrahedral complex

Isomerism in transition element complexes

• Transition element complexes can exhibit stereoisomerism
• Even though transition element complexes do not have a double bond, they can still have geometrical isomers
• Square planar and octahedral complexes with two pairs of different ligands exhibit cis-trans isomerism
• Examples of octahedral complexes that exhibit geometrical isomerism are the [Co(NH₃)₄(H₂O)₂]²⁺ and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ complexes
  • [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ can also be written as [Ni(en)₂(H₂O)₂]²⁺

• Like in the square planar complexes such as cis-platin and trans-platin, if the two ‘different’ ligands are next to each other then that is the ‘cis’ isomer, and if the two ‘different’ ligands are opposite each other then this is the ‘trans’ isomer
  • In [Co(NH₃)₄(H₂O)₂]²⁺, the two water ligands are next door to each other in the cis isomer and are opposite each other in the trans isomer

Octahedral transition metal complexes exhibiting geometrical isomerism

Optical isomerism

• Octahedral complexes with bidentate ligands also have optical isomers
• This means that the two forms are non-superimposable mirror images of each other
  • They have no plane of symmetry, and one image cannot be placed directly on top of the other

• The optical isomers only differ in their ability to rotate the plane of polarised light in opposite directions
• Examples of octahedral complexes that have optical isomers are the [Ni(H₂NCH₂CH₂NH₂)₃]²⁺and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ complexes
  • The ligand H₂NCH₂CH₂NH₂ can also be written as ‘en’ instead

Octahedral transition metal complexes exhibiting optical isomerism